This invention relates to flue gas desulfurization. More specifically, it relates to a regenerative process for the removal of sulfur dioxide from flue gas and other gas streams.
Present processes for the removal of sulfur dioxide from such gas streams as flue gas can be divided into two categories, namely throwaway processes and regenerative or recovery processes.
In the throwaway process, exemplified as in U.S. Pat. No. 4,060,587, the sulfur dioxide content of the gas stream is normally removed by contacting the stream with an aqueous slurry of lime or limestone, resulting in the formation of a sludge of mainly calcium sulfite. Such a process, while relatively simple and currently the most popular for flue gas desulfurization, has major drawbacks which include the economic and ecological problems attached to the disposal of the sludge, the necessity of continuously supplying fresh absorbent to the system, and numerous operational problems such as the tendency toward crust formation.
As a result of such drawbacks with the throwaway process, numerous regenerative processes, in which the absorbent is recycled to the process and the absorbed sulfur dioxide is recovered in the form of a usable product, have been devised. Among these are processes in which the absorbent is a buffered aqueous solution of sodium citrate. In one version of such citrate processes, described, for example, by Vasan in Chemical Engineering Progress, Vol. 71, No. 5, May, 1975, pages 61-65, the sulfur dioxide absorbed in the citrate solution is recovered as sulfur by contacting the sulfur dioxide-rich solution with hydrogen sulfide. This approach, however, has certain disadvantages, including the necessity of generating and handling hydrogen sulfide. In an alternative version, disclosed, for example, by Nissen et al in Proceedings: Symposium on Flue Gas Desulfurization--New Orleans, EPA-600/2-76-136b, May, 1976, pages 843-864, by Farrington et al in "The Flakt-Boliden Process for SO.sub.2 Recovery", presentation at the 1979 Annual Meeting of the Metallurgical Society of AIME, New Orleans, La., February, 1979, and by U.S. Pat. Nos. 2,031,802 and 4,140,751, the sulfur dioxide is recovered simply by stripping it, preferably with steam, directly from the absorbing solution. Such recovery, while eliminating the necessity of a regenerating chemical, does require considerable steam consumption.
It is therefore the object of the present invention to provide a regenerative process for the desulfurization of gases containing sulfur dioxide which has reduced chemical, operating and equipment costs over existing processes.
To date, attempts to use potassium salts in a regenerative process for the removal of sulfur dioxide from gas streams have not met with appreciable success. Such attempts are described in Slack and Hollinden, Sulfur Dioxide Removal from Waste Gases, 2nd Ed., Noyes Data Corporation, Park Ridge, N.J., 1975, pages 222-224, and in Advanced Concepts: SO.sub.2 Removal Process Improvements, EPA-600/7-78-216, November, 1978, pages 1-28. Sodium and/or potassium hydroxides and carbonates are indicated as the preferred alkalis for buffering in the regenerative process of U.S. Pat. No. 4,133,650, in which the absorbent is a buffered aqueous solution of a dicarboxylic acid.
A method for the selective removal of sulfur dioxide from a gas also containing hydrogen sulfide in which the gas is contacted with an aqueous solution of an alkali metal salt of an organic acid, preferably potassium citrate, is disclosed in U.S. Pat. Nos. 3,757,488 and 3,833,508. The method fails, however, to disclose or suggest regeneration of the absorbing solution or recovery of the absorbed sulfur dioxide, much less a means for such regeneration and recovery.
From a study of equilibrium vapor pressures of sulfur dioxide over aqueous sulfite-bisulfite solutions, Johnstone et al, in Industrial and Engineering Chemistry, Vol. 30, No. 1, January, 1938, pages 101-109, concluded that the effect of temperature on the equilibrium vapor pressure of sulfur dioxide over homogeneous solutions of salts of weak acids should not differ greatly from the alkali sulfite-bisulfite solutions, in which sodium and potassium were found to be comparable. The assumption of the essential equivalence of sodium and potassium in absorption systems employing citrate ions was again implied by Oestreich in Equilibrium Partial Pressure of Sulfur Dioxide in Alkaline Scrubbing Processes, EPA-600/2-76-279, October, 1976, and by Rochelle in Process Synthesis and Innovation in Flue Gas Desulfurization, EPRI FP-463-SR, Special Report, July, 1977, pages 4-37 thru 4-43.